1. Field of the Invention
The present invention relates to an oxygen concentration sensing apparatus, and more particularly to an oxygen concentration sensing apparatus for sensing an oxygen concentration in an exhaust gas of an internal combustion engine.
2. Description of Background Information
In order to accelerate the purification of the exhaust gas and to improve the fuel economy of an internal combustion engine, a feedback type air/fuel ratio control system is used, in which oxygen concentration in the exhaust gas is detected and air/fuel ratio of the mixture supplied to the engine is controlled to a target air/fuel ratio by a feedback control operation in accordance with a result of the detection of the oxygen concentration.
As an oxygen concentration sensor for use in such an air/fuel ratio control system, there is a type of sensor which is capable of producing an output signal whose level is proportional to the oxygen concentration in the exhaust gas of the engine, and the detail of which is disclosed in Japanese Patent Application laid open No. 58-153155. This oxygen concentration sensor includes an oxygen concentration sensing unit having a general construction including a pair of flat solid electrolyte members having oxygen ion permeability. These oxygen ion conductive solid electrolyte members are placed in the gas under measurement of the engine, and electrodes are respectively provided on the front and back surfaces of both of the solid electrolyte members. In other words, each pair of electrodes sandwich each solid electrolyte member. These two solid electrolyte members each having a pair of electrodes are arranged in parallel so as to face each other and forming a gap portion, or in other words, a restricted region between them.
With this arrangement, one of the solid electrolyte members serves as an oxygen pump element and the other one of the solid electrolyte members serves as a sensor cell element for sensing an oxygen concentration ratio. In an ambient atmosphere of the gas under measurement, a drive current is supplied across the electrodes of the oxygen pump element in such a manner that the electrode facing the gap portion operates as a negative electrode. By the supply of this current, i.e. a pump current, the oxygen component of the gas in the gap portion is ionized on the surface of the negative electrode of the oxygen pump element. The oxygen ions migrate through the inside of the oxygen pump element to the positive electrode, where the oxygen ions are released from the surface thereof in the form of the oxygen gas.
While this movement of the oxygen ions is taking place, the oxygen concentration becomes different for the gas in the gap portion and the gas outside the sensor cell element because of a decrease of the oxygen gas component in the gap portion. Therefore, if a current to the oxygen pump element, i.e. the pump current is maintained constant, a voltage proportional to the difference of the oxygen concentration, that is the oxygen concentration in the gas under measurement, develops across the electrodes of the sensor cell element. By this voltage generated at the sensor cell element, whether the air/fuel ratio of the mixture supplied to the engine is rich or lean with respect to a target air/fuel ratio can be determined. In the case of the air/fuel ratio control by means of the secondary air, the secondary air is supplied to the engine when the air/fuel ratio is detected to be rich. On the other hand, the supply of the secondary air is stopped when the air/fuel ratio is detected to be lean. In this way, the air/fuel ratio of the mixture is controlled toward the target air/fuel ratio. If the magnitude of the pump current supplied to the oxygen pump element is controlled so that the voltage generated across the sensor cell element is maintained constant, the magnitude of the pump current varies substantially in proportion to the oxygen concentration in the exhaust gas under a condition of a constant temperature. Thus, the pump current can be also used for the detection of the air/fuel ratio.
In this type of oxygen concentration sensor, if an excessive current is supplied to the oxygen pump element, it causes the blackening phenomenon by which the oxygen ions are removed from the solid electrolyte members. For instance, when zirconium dioxide (ZrO.sub.2) is used as the solid electrolyte, the oxygen ions O.sub.2 are taken from the zirconium dioxide (ZrO.sub.2) and zirconium (Zr) is separated out. As a result of this blackening phenomenon, the deterioration of the oxygen pump element takes place rapidly, to cause a debasement of the operation of the oxygen concentration sensor as a whole. Therefore, the pump current value must be controlled to be lower than values in a region of generation of the blackening phenomenon (blackening phenomenon generation region) so as to prevent the blackening phenomenon before it is generated.
FIG. 1 shows lines indicating the pump current to the oxygen pump element versus oxygen concentration relation and a boundary line of the generation of the blackening phenomenon with respect to different values of the voltage Vs developing at the sensor cell element, which voltage functions as a parameter. As illustrated, magnitude of the current I.sub.p varies in proportion to the oxygen concentration, and the rate of variation is different for several different values of the voltage V.sub.s. In other words, the voltage V.sub.s is a parameter which determines the relation between the magnitude of the current I.sub.p and the oxygen concentration. As illustrated in this figure, the boundary line of the generation of the blackening phenomenon is shown, like the relation between the pump current and the oxygen concentration, as a first-degree function of the oxygen concentration value. Therefore, whether or not the pump current value belongs to values in the blackening phenomenon generation region can be determined from the value of the voltage Vs. Therefore, if the voltage Vs exceeds a predetermined voltage, it can be considered that the value of the pump current is approaching to the region of the generation of the blackening phenomenon, and the generation of the blackening phenomenon can be prevented by reducing the pump current. However, it is recognized that the internal resistance of the oxygen pump element becomes large when the temperature of the oxygen pump element is low. Therefore, under such a condition, the pump current value generating the blackening phenomenon becomes lower than the case in which the temperature of the pump element is high. This means that there is a chance of the generation of the blackening phenomenon even if the voltage Vs is lower than the predetermined voltage.